1. Technical Field
The invention is directed to Light Emitting Diodes (LEDs) and, in particular, to LEDs that emit light in a pattern.
2. Art Background
Flat panel displays containing light emitting diodes are ubiquitous features of many products. Because of the need to minimize the manufacturing cost of most products, inexpensive ways to manufacture flat panel displays are of considerable interest. As noted in Lidzey, D. G., et al., xe2x80x9cPhotoprocessed and micropatterned conjugated polymer LEDs,xe2x80x9d Synthetic Metals, Vol. 82, pp. 141-148 (1996), organic materials have been investigated for use as the emissive layers in LEDs because large-area devices can be made cheaply and easily using such materials. Also, a greater variety of emission colors is obtained when organic emissive layers are used instead of inorganic emissive layers. LEDs with organic emissive layers have a greater electrical efficiency than comparable LEDs with an inorganic emissive layer.
LEDs are generally formed on transparent substrates such as glass or plastic. A light emitting material is sandwiched between an anode formed on the substrate and a cathode. When current is supplied to the anode, electrons and holes recombine in the light-emitting material sandwiched between the anode and the cathode. As a result of this recombination, light emits from the light-emitting material and through the transparent substrate.
One use for LEDs is in displays having a fixed pattern. In such displays, there are at least two areas of contrast when the display is on. The areas of contrast (e.g. light and dark) provide a desired picture (e.g., a logo) or message (e.g. an xe2x80x9cEXITxe2x80x9d sign). Such a patterned array of LEDs is described in the previously mentioned Lidzey et al. reference which was mentioned previously. A patterned cathode is formed over an emissive layer (poly(2,5-dialkoxy-p-phenylenevinylene). When a voltage is applied to the ITO anode, light is emitted in a pattern that corresponds to the cathode pattern, because light is only emitted from those portions of the emissive layer sandwiched between the anode and the cathode. A patterned display is also obtained by patterning the anode instead of the cathode.
However, there are certain limitations on the patterns that can be obtained by patterning the anode or the cathode. For example, a simple pattern such as the letter xe2x80x9cOxe2x80x9d is not easily obtained by patterning the cathode. This is because the mask used to form the pattern must be one integral unit. The letter xe2x80x9cOxe2x80x9d requires complete physical separation between the portion of the mask inside the xe2x80x9cOxe2x80x9d from the portion outside the xe2x80x9cO.xe2x80x9d Such a complete physical separation cannot be obtained in a single unit mask. There must be some physical connection between the portion of the mask inside the xe2x80x9cOxe2x80x9d and the portion of the mask outside the xe2x80x9cO.xe2x80x9d Furthermore, the expedients used to pattern the cathode in the manner described in Lidzey et al. degrade the organic emissive layer underlying the cathode.
Different restrictions are placed on a patterned anode such as indium tin oxide. For example, the conductivity of ITO is reduced when patterned into narrow lines. Therefore, the brightness of the display is not evenly distributed if a narrow portion of ITO is required by the pattern. Furthermore, the ITO must be electrically interconnected and therefore a pattern that is not continuous is not practicable.
In response to the limitations imposed by patterning anodes and cathodes, Renak, M., et al., Microlithographic Process for Patterning Conjugated Emissive Polymers,xe2x80x9d Advanced Materials, Vol. 9, No. 5, pp. 392-395 (1995) describes a patterned LED display in which the electron emissive layer (poly(p-phenylenevinylene)) is patterned. Renak et al. describes a device in which the patterned layer of poly(p-phenylenevinylene) (PPV) is formed over an ITO layer. An electron transport layer was cast over the PPV layer. A cathode was formed over the electron transport layer. The electron transport layer is present to prevent direct electrical contact between the ITO anode and the cathode.
When a voltage is applied to the ITO of the device described in Renak et al., light is emitted from the patterned PPV layer in the pattern of the PPV layer. However, the approach does not afford much flexibility, as the only contrast provided by such a display is the contrast between the PPV area of the display (which emits light when the device is on) and the non-PPV area of the display (which does not emit light even when the device is on). Thus the basis for contrast in such a display is basically either on or off. Furthermore, Renak et al requires the use of light-emitting polymers that are also photosensitive in order to pattern the light-emitting layer. Thus, the choices for the light-emitting material for the Renak et al. device are extremely limited.
A display that provides the potential for a greater variety of visual contrast, yet does not require that either the anode or the cathode be patterned, is desired.
LED devices have a layer or layers of active material sandwiched between an anode and a cathode. Active layers, as used herein are layers of material in which either electron transport, hole transport, light emission, or some combination thereof, occur. The present invention is directed to an LED device in which at least one of the active layers is patterned to have at least a first thickness and a second thickness. The patterned organic layer is sandwiched between an anode and a cathode. When the LED device is on (i.e. when sufficient current is provided to the anode to induce electron/hole recombination in the light emitting layer) there is a visually perceivable contrast between the portion of the LED device that corresponds to the active layer of the first thickness and the portion of the LED device that corresponds to the active layer having the second thickness.
The active layer is one or more layers of organic material. In one embodiment, the active layer is a patterned layer of a material in which electron/hole recombination and, thus, light emission occurs. In a second embodiment, the active layer is a combination of two layers: a layer of material in which light emission occurs coupled with a hole transport or electron transport layer. The hole transport layer, if present, is in contact with the anode. The electron transport layer, if present, is in contact with the cathode. In the second embodiment, the aggregate thickness of the active layer (i.e. the combined thickness of the light emitting layer and the hole transport or electron transport layer) is not uniform because one of either the light emitting layer and the electron transport layer or the hole transport layer is patterned. An active layer consisting of a patterned electron transport layer formed on a layer of light emitting material of uniform thickness is one example.
In a third embodiment both the light emitting layer and the hole transport or electron transport layer are patterned. However, the patterns are complimentary (the thinner portion of one layer is aligned with the thicker portion of the other layer and vice-versa) so that the aggregate thickness of the two layers is uniform.
As a result of the one or more patterned layers in the active layer, the LED device emits light through one portion associated with a first layer thickness that is visually distinct from a second portion of the LED device associated with a second layer thickness. In the context of the present invention, the thickness that is referred to is the thickness of the patterned layer and not the aggregate thickness of the active layers. In one embodiment of the present invention, when the LED is on, the LED device only emits light through the portion associated with the thinner portion of the patterned light-emitting layer and not through the second portion associated with the thicker portion of the light-emitting layer. In an alternate embodiment, the LED device emits light of a first color through the first portion and light of a second color through the second portion. In yet another embodiment, the LED device emits light of a first intensity through the first portion and light of a second intensity through the second portion. In this embodiment, the difference between the first intensity and the second intensity is visually perceivable.
It is also contemplated that the patterned layer has more than two thicknesses. When the LED device is on, the region of the LED device associated with a particular thickness of the patterned layer is visually distinct from the other regions associated with the other thicknesses. The LED device of the present invention provides the flexibility to produce LED devices in a variety of patterns. The LED devices of the present invention are produced at low cost because of the ease in which a patterned layer is formed.
Since the light-emitting material of the LED of the present invention is sandwiched between an anode and a cathode, one of either the anode or the cathode is transparent to the emitted light so that the light emission is observable. Typically one of either the anode or the cathode is formed on a substrate. If the transparent anode is formed on a substrate, then the substrate on which the anode is formed is also transparent. Similarly, if the transparent cathode is formed on a substrate, then the substrate on which the cathode is formed is also transparent. For convenience, in the embodiments described herein, the anode is formed on the substrate. However, the present invention contemplates that the patterned active layer of the device of the present invention can also be placed between a cathode formed on a substrate and an anode. Furthermore, in certain embodiments the anode or the cathode is the substrate.
In one embodiment of the present invention, the LED device is formed by spinning a precursor of the organic light emitting material on a transparent substrate with an anode formed thereon. It is advantageous if the precursor is soluble in an organic solvent such as methanol. A mold is used to form the layer of precursor into a desired pattern. An elastomeric mold that has a first surface with a recessed portion in the desired pattern is one example of a suitable mold. The recessed portion functions as a channel for the precursor when the mold surface is placed in contact with the layer of organic light emitting material.
The mold surface is wetted with an organic solvent. When the wetted surface contacts the precursor, the precursor is partially dissolved and the dissolved precursor fills the channels in the mold. The solvent is evaporated and the mold is removed. After the mold is removed, the precursor layer has a surface relief pattern that corresponds to the pattern in the mold. In order to form a light emitting layer with three or more thicknesses, a mold that has channels with more than one depth is contemplated as suitable.
In the context of the present invention, any light emitting material that can be formed on a substrate in a desired pattern is contemplated as suitable. A precursor of poly(p-phenylene vinylene) is one example of a suitable material.